What is the functional cusp on an upper tooth?
Palatal cusp
What is the non functional cusp on an upper tooth?
Buccal cusp
What is the functional cusp on a lower tooth?
Buccal cusp
What is the non functional cusp on a lower tooth?
Lingual cusp
What is the non functional cusp reduction of a FGC?
1mm
What is the functional cusp reduction for a FGC?
1.5mm
Which burr would you use for occlusal reduction of an FGC?
554 burr
Which cusp bevel reduction is larger?
Larger functional cusp bevel and smaller non functional cusp bevel
When would you consider adding retentive features to a prep?
On teeth with short clinic crowns
What would you use to polish a crown prep?
White or green stones
What is a crown?
A restoration that encompasses coronal tooth tissue to cover any remaining tooth substance and any remaining restorations
What are the indications for a crown?
- Protection of weakened tooth tissue, e.g. RCT
- Fractured cusps
- Cracked tooth syndrome
- Maintenance of occlusion (prevention of overeruption of missing tooth)
- Replacement of failing crown
- Aesthetics
What must be considered when deciding to place a crown?
The provision of a crown must provide strength and protection that outweighs any weakening of the tooth caused by its preparation
What are contraindications of the crown?
- Inadequate coronal tooth tissue
- Poor OH
- Untreated primary disease, e.g. caries, per, periapical disease.
- Inadequate periodontal support
- Patient choice.
What must you do before the provision of a crown?
- PA to check for:
- Caries
- PA path (infection/occlusal trauma)
- Previous RCTs
- Size of pulp chamber
- Alveolar bone levels - periodontal
- Crown-root ratio
- Presence of ++ or any other path
- Sensibility testing
When would you use diagnostic casts?
When you intend to place multiple crowns to diagnose occlusal issues and for the formulation of a definitive treatment plan
What would you use to mount diagnostic casts?
Semi-adjustable articulator
What are the advantages of a diagnostic cast?
- Unobstructed view of the mouth from all aspects
- Occlusion can be viewed from the lingual aspect and discrepancies can be identified
- The length of teeth can be accurately assessed to determine what preps designs will improve adequate resistance and retention form.
- Incline, drifting of teeth, rotation and buccolingual displacement of teeth are clearly visible.
What are the biomechanics principles of crown preps?
- Preservation of tooth structure
- Retention and resistance form
- Structural durability of the restoration (material thickness)
- Marginal integrity
- Preservation of the periodontium
What can excessive removal of tooth structure cause?
- Pulpal inflammation (pain)
- Pulpal necrosis (RCT - 15% of teeth prepped for crowns)
- Acceleration of the restorative cycle (when a crown fails, a replacement crown is no longer an option)
- Reduced retention/resistance - reduced longevity
What is retention?
Retention is the ability of the preparation to prevent the removal of the restoration along its path of insertion
What is resistance?
Resistance is the ability of the preparation to prevent dislodging of the restoration by forces directed in an apical oblique or functional direction.
Why are resistance and retention important?
So the restoration can withstand the dislodging forces that it will encounter during function.
Which operator factors influence retention?
- Degree of taper (more parallel walls = greater retention)
- Length of preparation
- Type of cement selected
- Total surface area of the cement film
- Area of cement under shear
- Roughness of tooth surface
